The present invention relates to a controller and, more particularly, to a controller that can achieve multi-variable controls through a single-variable control unit.
FIG. 1 shows a schematic diagram of a dynamic process, and three or more process variables are included. The process variables may change with time, and may affect each other. Also, in a dynamic process, there are at least one independent variable and at least one dependent variable.
FIG. 2 shows a schematic diagram of a dynamic process with a single-variable controller. As shown, for controlling a dependent variable, a controlled variable (CV) is defined, and an SISO (single-input-single-output) control unit 101 is applied to the dynamic process to form a control loop. After the control unit 101 is applied to the dynamic process, the controlled variable (CV) can be controlled at a certain reference value or setpoint (SP) through an independent variable (called a manipulated variable, MV) of the dynamic process that affects the controlled variable (CV). In FIG. 2, the solid lines in the diagram indicate the input and output signals of the control unit 101 and the signal directions. The thin dashed line between two variables in the diagram indicates that one variable will affect the other associated variable. The thick dashed lines between multiple variables in the diagram indicate that one variable directly or indirectly affects the other associated variable. The arrows indicate the influence directions.
Industrial processes are usually controlled by using single-variable controllers due to the technology maturity, simplicity, and low cost. However, most of industrial processes require more than one process variables to be controlled. In control technology of the prior art, there are other control methodologies, such as cascade control, feed-forward control, and multi-variable control, being developed, which can expand the applications of single-variable controllers so that they can be used in industrial processes requiring control of multiple process variables. Conventionally, when solving an industrial process requiring control of multiple process variables, multiple single-variable controllers are usually employed, with each single-variable controller used for controlling a controlled variable. Namely, each single-variable controller has one individual control loop consisting of one pair of a MV and a CV, and thus can control only one process variable (i.e. the CV) and cannot use other process variables to facilitate controlling the industrial process.
Therefore, it is deserved to develop a controller, which can achieve multi-variable controls through a single-variable control unit, and especially, a single-variable controller, which can achieve multi-variable or multi-function control.